Square metal tubing is used in a wide variety of applications. It is used, for example, in the construction of rollover and falling object protection systems (ROPS and FOPS, respectively) on heavy machinery, such as forklifts. Metal tubing is also widely used in all manner of railings, fences, brush-guards, building construction, etc.
Such applications often require corners or angled joints to be made between substantially straight sections of tubing. This is commonly accomplished in the prior art, for example, by welding together the ends of two straight metal tubes, or by bending the tubing to achieve the desired angle or curve (see, for example, FIGS. 1-5). However, both of these approaches suffer from several drawbacks.
The first drawback is that welding together two straight sections of tube results in a corner, which may not be aesthetically acceptable if a rounded look is desired. In addition, sharp corners present a hazard in that they are sharp points on which people and objects may get caught.
A drawback of bending square tubing to achieve the desired curve is that the minimum radius of the bend is limited. Short radius bends or elbows cannot be achieved by bending metal tubing because the side that forms the outer curve of the tube is stretched and weakened and may even be broken. In addition, the side that forms the inner curve of the tube is wrinkled or deformed such that its appearance and structural strength are compromised. Accordingly, only relatively large radius curves can be achieved by bending.
Furthermore, for FOPS and ROPS type applications, where the strength of the finished rectangular tube structure is crucial, weak joints are unacceptable. In some cases the weakness of bent joints and elbows can be compensated for by using thicker metal, however, this is in exchange for greater cost, labour and material. In addition, thicker metal results in a heavier structure, which may not be desirable. It appears, from commercially available bent tubing products, that the minimum radius that can be achieved by bending square steel tubing is approximately 2.5 times the width of the tubing (width being measured in a plane parallel to the radius of curvature). This limit will obviously vary with tube size, tube wall thickness, and bending techniques, etc.
In addition, bending is simply not practical for larger sized metal tubing. For example, 12″×12″ square steel tubing can hardly be bent at all (let alone to a curve having a radius of 30″ (2.5 times the width of the tubing)) simply because the tube will suffer excessive stretching and compression on the outer and inner faces of the curve, respectively.
In order to withstand the stretching caused by bending, metal tubes having a certain minimum thickness must be used. If the metal used is too thin, for a bend of given radius, it will break or be weakened to the point where it becomes structurally useless. In essence, the sharper the bend and/or the larger the width of the metal tube, the thicker the metal that must be used. Therefore for many applications, such as hand railings, a thinner metal could be used but for the requirement that it withstand the stress of bending.
Attempts have been made to address some of these needs in the prior art. For example, U.S. Pat. No. 5,441,241, issued to McKim, discloses a Knuckle for Welding of Safety Hand Railings. However, the knuckle disclosed by McKim being a solid piece of metal is inappropriate for use with large diameter rectangular tubing because it would be extremely heavy. In addition, McKim's knuckle results in a sharp inside angle or corner in which, in the case of handrails, clothing or even user's hands can be caught. Finally, the angle of McKim's knuckle cannot be modified, for example, on a job site during construction (i.e. each knuckle is manufactured for a specific angle and is not readily modified).
Several U.S. patents have issued for inventions relating to joints for structures, railings or fences, (for example, U.S. Pat. Nos. 4,667,935; 5,820,289; 2,930,638; 4,322,176; 5,617,694; 6,164,706; and 4,314,861) however, the systems disclosed by such patents generally suffer from one or more of the following disadvantages:
(a) they do not provide the requisite strength necessary for applications such as FOPS and ROPS;
(b) the angle of the joint or elbow cannot be readily adjusted or modified;
(c) they are not aesthetically pleasing;
(d) they are unnecessarily complex and/or expensive to produce; and
(e) they are limited in the radius and degree of bend that can be achieved.
For example, Kirschenmann et al., (U.S. Pat. No. 5,630,622) disclose a welded metal structure incorporating right-angled cast corner elements.
The method disclosed by Kirschenmann et al. does not conform to the standards set by the American Welding Society, or AWS (and similar organizations in other jurisdictions, such as the Canadian Welding Bureau) and they are prone to cracking, which can lead to catastrophic failure of structures such as ROPS and FOPS. The weld joint of Kirschenmann et al. is a butt-weld or butt-joint between two tubular members. The AWS Structural Welding Code requires that transitions between two butt welded members having different thicknesses must have smooth transitions having a slope of no more than 1 in 2.5 (i.e. it must be sloped, chamfered and/or tapered). The welded joint disclosed by Kirschenmann et al. does not conform to the standard set in the AWS Structural Code. Therefore, before such a weld/joint can be used, a sample must be prepared and submitted for testing. If the weld/joint is approved, then it can be incorporated into the structure. This is obviously a severe drawback of the invention of Kirschenmann et al., which results in substantial delays and added expense.
Further, in Kirschenmann et al. the ends of the straight structural members are not welded to the ends of the corner members. Rather, a “transition portion” of the corner members is welded to the straight tubular members. The transition portion is not the end of the cast corner member because of gussets that extend well beyond it. The gussets of Kirschenmann et al. are prone to cracking in the area proximate the base of the gussets due to the stress caused by the adjacent weld, (e.g. lamellar tearing). Referring to FIG. 15, the area 200 proximate the weld 210 is susceptible to cracking due to the drastic change in thickness of the corner member 220 (identified by reference number 20 in Kirschenmann et al.) proximate the weld. Therefore, at best, the gussets of Kirschenmann et al., provide negligible reinforcement and, at worst, result in a compromised welded joint due to its susceptibility to cracking.
In addition, the invention of Kirschenmann et al, is not amenable to modification. In other words, once the corner member of Kirschenmann et al. is manufactured it cannot be modified easily in order to, for example, adjust its angle. Other prior art methods of welding tubular metal structures also suffer from this disadvantage. In effect, if one attempts to change the angle of a corner member by cutting any portion of the corner member, then the cross-section of the end of the corner member will have been necessarily changed as well. Therefore the corner member can no longer be welded to other members structural members without taking additional time- and material-consuming steps to adapt the cross-sectional shape of the corner member to that of the other structural members. By way of example, FIG. 12 shows a prior art corner member 300. FIG. 13 shows the cross-section 330 of corner member 300 at the ends 310 (generally equivalent to the corner members 20 of Kirschenmann et al.). The ends 310 have 2″ diameters. Clearly, if either end 310 of corner member 300 is welded to another tubular structural member, the tubular structural member has to have a cross-section matching the ends 310. However, if the corner member 300 is modified by cutting it along a diagonal line, the cross section will change. The corner member can only then be welded to another structural tubular member having a similar cross-section. By way of example, FIG. 14 shows the cross section 340 of corner member 300 when it is cut along line 320. Referring to FIGS. 12-14, the cross-sections of FIGS. 13 and 14 differ substantially (i.e. the diameter increases by almost 50%, to 2⅞″).
Accordingly, there exists a need in the art for elbows and joints for use with metal tubing that address these deficiencies.